码迷,mamicode.com
首页 > 其他好文 > 详细

block 解析 - block变量

时间:2014-07-29 10:30:16      阅读:287      评论:0      收藏:0      [点我收藏+]

标签:des   style   blog   http   color   os   strong   for   

block变量

上一篇 讲的是block静态变量的特性,这里我们来看一下_block变量。引用官方:

You can specify that an imported variable be mutable—that is, read-write— by applying the __block storage type modifier. __blockstorage is similar to, but mutually exclusive of, the registerauto, and static storage types for local variables.

通过指定__block存储类型修饰符,可以读写。__block存储是类似的,但相互排斥的,寄存器,自动变量,和局部变量和静态变量类型(后面这句不太理解)。 我们来写个例子:
void main1()
{
    __block char *_para1="a";
    printf("init _para1:%s,%p,%p\n",_para1,_para1,&_para1);
    void(^testBlock)(void)=^{
        printf("exute _para1:%s,%p,%p\n",_para1,_para1,&_para1);
    };
    _para1="b";
    printf("before _para1:%s,%p,%p\n",_para1,_para1,&_para1);
    testBlock();
    printf("after _para1:%s,%p,%p\n",_para1,_para1,&_para1);
}
执行后输出如下:
init _para1:a,0x47f4,0xbfffc9c0
before _para1:b,0x4829,0x8da4580
exute _para1:b,0x4829,0x8da4580
after _para1:b,0x4829,0x8da4580

这里加了__block修饰符。通过日志可以看出,block内部_para1的地址、值和执行前的_para1一样,在block初始化后,对变量_para1的修改,可以同步到block内,block内并不是截获了变量的值。我们可以看下通过clang 转换后的后的cpp代码:

//block 的实现函数的对象
struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};
//block 引用的参数对象(每一个参数生成一个结构体)
struct __Block_byref__para1_0 {
  void *__isa;
__Block_byref__para1_0 *__forwarding;
 int __flags;
 int __size;
 char *_para1;
};
//block对象
struct __main1_block_impl_0 {
  struct __block_impl impl;
  struct __main1_block_desc_0* Desc;
  __Block_byref__para1_0 *_para1; // by ref
  __main1_block_impl_0(void *fp, struct __main1_block_desc_0 *desc, __Block_byref__para1_0 *__para1, int flags=0) : _para1(__para1->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
//block 的实现
static void __main1_block_func_0(struct __main1_block_impl_0 *__cself) {
  __Block_byref__para1_0 *_para1 = __cself->_para1; // bound by ref

        printf("exute _para1:%s,%p,%p\n",(_para1->__forwarding->_para1),(_para1->__forwarding->_para1),&(_para1->__forwarding->_para1));
    }
//其他函数(copy、dispose:为了管理_Block_byref__para1_0 结构体变量的内存)
static void __main1_block_copy_0(struct __main1_block_impl_0*dst, struct __main1_block_impl_0*src) {_Block_object_assign((void*)&dst->_para1, (void*)src->_para1, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main1_block_dispose_0(struct __main1_block_impl_0*src) {_Block_object_dispose((void*)src->_para1, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main1_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main1_block_impl_0*, struct __main1_block_impl_0*);
  void (*dispose)(struct __main1_block_impl_0*);
} __main1_block_desc_0_DATA = { 0, sizeof(struct __main1_block_impl_0), __main1_block_copy_0, __main1_block_dispose_0};
//这是我们的测试函数
void main1()
{
    __attribute__((__blocks__(byref))) __Block_byref__para1_0 _para1 = {(void*)0,(__Block_byref__para1_0 *)&_para1, 0, sizeof(__Block_byref__para1_0), "a"};
    printf("init _para1:%s,%p,%p\n",(_para1.__forwarding->_para1),(_para1.__forwarding->_para1),&(_para1.__forwarding->_para1));
    void(*testBlock)(void)=(void (*)())&__main1_block_impl_0((void *)__main1_block_func_0, &__main1_block_desc_0_DATA, (__Block_byref__para1_0 *)&_para1, 570425344);
    (_para1.__forwarding->_para1)="b";
    printf("before _para1:%s,%p,%p\n",(_para1.__forwarding->_para1),(_para1.__forwarding->_para1),&(_para1.__forwarding->_para1));
    ((void (*)(__block_impl *))((__block_impl *)testBlock)->FuncPtr)((__block_impl *)testBlock);
    printf("after _para1:%s,%p,%p\n",(_para1.__forwarding->_para1),(_para1.__forwarding->_para1),&(_para1.__forwarding->_para1));
}

发现和不加__block不太一样,多了一个结构体(_Block_byref__para1_0)


struct __Block_byref__para1_0 {
  void *__isa;
__Block_byref__para1_0 *__forwarding;
 int __flags;
 int __size;
 char *_para1;
};

这个结构体就是用__block 声明的变量,声明__block变量_para1 转换后的代码如下:

__attribute__((__blocks__(byref))) __Block_byref__para1_0 _para1 = {(void*)0,(__Block_byref__para1_0 *)&_para1, 0, sizeof(__Block_byref__para1_0), "a"};

其实就是生成一个_Block_byref__para1_0对象,__forwarding 指针指向变量结构体自己

void(*testBlock)(void)=(void (*)())&__main1_block_impl_0((void *)__main1_block_func_0, &__main1_block_desc_0_DATA, (__Block_byref__para1_0 *)&_para1, 570425344);

初始化block,传递结构体变量_para1、函数地址、描述信息等参数,block对象的成员 _para1 引用了_para1结构体的地址,这样就可以修改_para1,而且外界对_para1结构体的修改都可以同步到block对象的成员_para1中,修改代码如下:

(_para1.__forwarding->_para1)="b";

直接通过__Block_byref__para1_0 结构体的成员__forwarding(指向自己)取得_para1成员的地址,然后就可以读写,达到了修改变量的目的。

((void (*)(__block_impl *))((__block_impl *)testBlock)->FuncPtr)((__block_impl *)testBlock);

执行block,依然是通过调用函数指针 FuncPtr 实现,并传递block自身。函数体代码如下:

static void __main1_block_func_0(struct __main1_block_impl_0 *__cself) {
  __Block_byref__para1_0 *_para1 = __cself->_para1; // bound by ref

        printf("exute _para1:%s,%p,%p\n",(_para1->__forwarding->_para1),(_para1->__forwarding->_para1),&(_para1->__forwarding->_para1));
    }

在函数体内,通过block对象的成员_para1(__Block_byref__para1_0的指针,再通过自身的成员__forwarding指针来获取_para1。

我的理解就是指针引用的关系,不知道为什么要加一个__forwarding指针 ,直接去引用自身的成员 _para1难道不行吗?希望大神赐教。

block 解析 - block变量,布布扣,bubuko.com

block 解析 - block变量

标签:des   style   blog   http   color   os   strong   for   

原文地址:http://www.cnblogs.com/NarutoYq/p/3873194.html

(0)
(0)
   
举报
评论 一句话评论(0
登录后才能评论!
© 2014 mamicode.com 版权所有  联系我们:gaon5@hotmail.com
迷上了代码!